DECKGL integration - Phase 1 (#3771)

* DECKGL integration

Adding a new set of geospatial visualizations building on top of the
awesome deck.gl library. https://github.com/uber/deck.gl

While the end goal it to expose all types of layers and let users bind
their data to control most props exposed by the deck.gl API, this
PR focusses on a first set of visualizations and props:

* ScatterLayer
* HexagonLayer
* GridLayer
* ScreenGridLayer

* Addressing comments

* lint

* Linting

* Addressing chri's comments

superset/assets/javascripts/explore/components/Control.jsx

@@ -21,6 +21,7 @@ const propTypes = {
   value: PropTypes.oneOfType([
     PropTypes.string,
     PropTypes.number,
+    PropTypes.object,
     PropTypes.bool,
     PropTypes.array,
     PropTypes.func]),